0116. Area

Input file name:	area.ln
Output file name:	area.out
Time limit:	1 s
Memory limit:	64 megabytes

Calculate the area of a polygon.

Input file

First line contains integer number n (3 ≤ n ≤ 50000) – number of vertices. n lines follow, containing two integer numbers x and y each (-10000 ≤ x,y ≤ 10000) – coordinates of vertices.

The vertices indeed form a polygon, i.e. the edges neither cross, nor touch each other.

Vertices are given in counterclockwise order.

Output file

Output the area of the polygon with six hundred digits after decimal point.

Examples:

Output file contains six hundred zeroes.

area.ln	area.out
3 0 0 1 0 0 1	0.5000000000...000

Source: Petrozavodsk Summer 2003. Blitz Kontest, Monday, August 25
Author: Andrew Lopatin, Nick Durov

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